It is well known in the plastic molding art to use pressurized fluid in conjunction with the injection molding of articles. The pressurized fluid is typically nitrogen gas which is introduced into the molten plastic.
Pressurized fluid serves many purposes. First, it allows the article so formed to have hollow interior portions which result in weight and material savings. Secondly, it minimizes the molded-in stresses by eliminating high second stage injection pressure. This also reduces part weight in that the gas is used to fill out the part. Thirdly, the pressurized fluid applies outward pressure to force the plastic against the surfaces of the mold cavity while the article is setting up. This enhances surface quality by eliminating sink marks, especially in areas of the article having thicker plastic sections, such as structural ribs or bosses.
One problem associated with fluid assisted injection molding is that the gas and the plastic are confluent within the injection nozzle. This causes the plastic within the nozzle to become contaminated with the gas. This is highly undesirable since a later injection of plastic into the mold contains contaminated plastic which results in poor quality. In other words, the gas mixes with the molten plastic in the nozzle and the resulting resin affects the quality of the product surface.
One prior art attempt to solve this problem is disclosed in the Yasuike U.S. Pat. No. 4,106,887 which discloses an injection molding nozzle similar to the injection molding nozzle disclosed in the Kataoka U.S. Pat. No. 4,140,672. A check valve is disposed within a longitudinally extending fluid passage to prevent the injected material from reversing. The problem with such prior art is insufficient force required to completely close the nozzle, especially when a relatively high viscous plastic is injected by the nozzle.
The Friederich U.S. Pat. No. 4,101,617 discloses a method for injection molding of hollow-shaped bodies including a nozzle for introducing both gas and molten resin into a mold. A valve is reciprocable within the nozzle to control the flow of gas and molten resin into the mold.
The Hendry U.S. Pat. No. 4,333,608 discloses an injection molding nozzle, including a shut-off plunger, which is operated by a fluid and which is opened by the flow of plasticized material through the nozzle. A control rod external to the nozzle controls the movement of the shut-off plunger and is adjustable so that the amount of plasticized resin flowing through the nozzle can be controlled.
The Kataoka U.S. Pat. No. 4,140,672 discloses an injection molding nozzle and method wherein a needle-type nozzle is provided with a central duct for injecting viscous liquid into a mold cavity.
The Yasuike U.S. Pat. No. 3,966,372 discloses a nozzle including an injection cylinder having a movable mandrel contained therein. The mandrel divides the bore of the cylinder into two different chambers. The mandrel has a central passage which extends axially therethrough so that the rear and front chambers are communicated. First and second thermoplastic resins are charged into the nozzle and are consequently injected in a mass into a mold cavity.
The Yasuike U.S. Pat. No. 4,129,635 discloses an injecting cylinder including a nozzle portion. The nozzle portion includes an injecting passage within which is located a gas charging device which, in turn, is provided with a charging nozzle which opens through the injecting passage.